The present invention relates generally to optical fibers and, more particularly, to optical fibers having an increased threshold for stimulated Brillouin scattering.
In many applications, such as high power fiber optic industrial lasers and scaleable high power fiber optic phased array laser systems, it is desirable to transmit optical signals having substantial amounts of power via optical fibers. Unfortunately, stimulated Brillouin scattering oftentimes limits the amount of power that can be transmitted via an optical fiber such that, even as additional input power is provided, the output power remains relatively fixed at the threshold at which stimulated Brillouin scattering commences.
In general, stimulated Brillouin scattering is a phase-matched parametric amplification process involving the coupling of a optical wave, an acoustic wave and a backward propagating Stokes wave. In this regard, variations in the index of refraction of an optical fiber induced by pressure differences created by an acoustic wave traveling along the optical fiber can cause a portion of the optical wave to be backscattered, thereby creating the backward propagating Stokes wave. The backward propagating Stokes wave essentially robs power from the optical wave so as to limit the power of the optical signals that can be transmitted via the optical fiber. With reference to quantum physics, stimulated Brillouin scattering can therefore be described by the transfer of a photon from the optical wave into a new Stokes photon of lower frequency and the creation of a new phonon that adds to the acoustic wave.
With reference to FIG. 1, as the input power of the signal transmitted via an optical fiber is increased up to the threshold for stimulated Brillouin scattering, the power level of the signals output by the optical fiber similarly increases as evidenced by positive slope of curve 10. Upon reaching the threshold for stimulated Brillouin scattering, however, further increases in the power of the signals transmitted via the optical fiber will not translate into increased power levels of the optical signals output by the optical fiber. Instead, the power level of the optical signals output via the optical fiber will remain at the threshold at which stimulated Brillouin scattering commences as evidenced by the horizontal portion of curve 10, while the additional input power will be transferred to the backward propagating Stokes wave as shown by the positive slope of curve 12.
Parametric processes, such as stimulated Brillouin scattering, are enhanced in guided wave structures in general, and optical fibers in particular, because the waves that interact, i.e., the optical waves, the acoustic waves, and the Stokes waves, are maintained in the core over relatively long distances. Moreover, stimulated Brillouin scattering is particularly apparent in optical fibers that exhibit a significant overlap of the fundamental optical and acoustic modes within the core of the optical fiber. In this regard, an overlap integral is defined as the integral of the product of the acoustic wave amplitude and the optical wave amplitude over the lateral cross-sectional area of the optical fiber. As the overlap integral approaches unity, coupling between the optical waves and the acoustic waves is at a maximum, thereby resulting in a high level of stimulated Brillouin scattering. As depicted in FIG. 2, for example, a conventional optical fiber having a core doped with GeO2 is susceptible to the early onset of stimulated Brillouin scattering since the fundamental optical and acoustic modes have a 67% mode overlap in the core for an optical wavelength of 1.55 microns and an acoustic frequency of 11.25 GHz. Thus, the forward propagating optical wave of such an optical fiber will couple energy into the formation of a longitudinal acoustic wave which, in turn, can reflect a portion of the power carried by the optical wave back toward the source.
In order to avoid the limitations imposed by the threshold at which stimulated Brillouin scattering commences, optical systems are typically designed such that the optical fibers are operated below the threshold for the onset of stimulated Brillouin scattering. As will be apparent, this approach effectively limits the performance and scalability of the optical systems and may effectively prevent the optical system from being utilized for applications demanding high energy levels. Alternatively, some optical systems utilize a plurality of optical fibers such that the total power handling capability of the plurality of optical fibers satisfies the power requirements of the particular application while ensuring that the power of the optical signals transmitted via each optical fiber is below the threshold at which stimulated Brillouin scattering commences. While facilitating the delivery of optical signals having increased power levels, optical systems of this type obviously include an increased number of components, thereby leading to increased costs and increased weight and volume requirements. Thus, it would be desirable to provide an improved technique for optically transmitting relatively large amounts of power, such as power levels that exceed the threshold at which stimulated Brillouin scattering would commence in a typical optical fiber, such that lasers and other high energy optical systems can be developed without requiring the use of multiple optical fibers that unnecessarily increase the weight and volume of the optical system.
An optical fiber having an elevated threshold for stimulated Brillouin scattering is therefore provided. The optical fiber includes a longitudinally extending core and a cladding surrounding the core and extending lengthwise therealong, wherein both the core and the cladding are specifically designed to guide optical waves through the core while anti-guiding acoustic waves. Moreover, the optical fiber includes other features to alter the mode profile of the acoustic waves and/or to further promote the lateral radiation of at least some of the acoustic waves. The threshold for stimulated Brillouin scattering can therefore be increased relative to a conventional optical fiber since the forward propagating optical wave cannot couple energy into the formation of a longitudinal acoustic wave as readily as in conventional optical fibers due to the anti-guiding of the acoustic waves and the alterations of the mode profile.
The core of the optical fiber of the present invention has a first index of refraction and a first acoustic wave propagation velocity. Similarly, the cladding has a second index of refraction that is less than the first index of refraction of the core and a second acoustic wave propagation velocity that is less than the first acoustic wave propagation velocity of the core. In order for the core and the cladding to have indices of refraction and acoustic wave velocities with the proper relationship, the optical fiber of one embodiment has a core that includes aluminum oxide as a dopant and/or a cladding that includes fluorine or boron oxide as a dopant. As a result of the relationship of the indices of refraction and acoustic wave velocities, optical waves can be guided through the core, while the acoustic waves are radiated away from the core and into the cladding, i.e., the acoustic waves are anti-guided. Due to the guiding of the optical waves and the anti-guiding of the acoustic waves, the fundamental optical and acoustic modes will not overlap as much within the core as in conventional optical fibers and the threshold for stimulated Brillouin scattering will be accordingly increased.
In one embodiment, the optical fiber further includes an irregular coating disposed on the cladding that varies in a lengthwise direction in order to alter the mode profile of the acoustic waves. For example, the irregular coating can be an acoustically dampening material that is acoustically matched to the cladding. As such, acoustic waves that reach the interface of the cladding and the coating will continue to radiate laterally from the cladding into the coating for further dampening. In order to couple the fundamental acoustic mode into higher order acoustic modes which provide little, if any, power to the stimulated Brillouin scattering process and to incoherently scatter acoustic energy back into the cladding and the core, the coating is irregular. For example, the coating can have a lateral thickness that varies randomly in a lengthwise direction. Alternatively, the coating can have a density that varies randomly in a lengthwise direction. Further, the coating can include a plurality of segments having different lengths that are spaced apart in a lengthwise direction. Still further, the coating can include a plurality of segments that are spaced apart in a lengthwise direction by gaps of different lengths. Regardless of the type of irregularity, the coating is designed to change the mode profile of the acoustic waves that reach the interface between the cladding and the coating such that any acoustic waves that are scattered back into the cladding and/or the core will have a negligible influence on the optical waves guided through the core.
According to another embodiment, the optical fiber includes a core and a cladding as described above along with a quarter wave layer disposed on and extending lengthwise along the cladding. In order to promote the lateral or radial radiation of the acoustic waves that reach the interface between the cladding and the quarter wave layer, the quarter wave layer has a thickness that equals an odd multiple of a quarter of a predetermined Brillouin scattering wavelength. By promoting the radial or lateral radiation of the acoustic waves away from the core and cladding of the optical fiber, the threshold at which stimulated Brillouin scattering commences is further increased.
In order to further alter the mode profile of the acoustic waves, the cladding can have a lateral thickness that varies irregularly in a lengthwise direction. Thus, any acoustic waves reflected from one location of the exterior surface of the cladding back toward the core will be out of phase from acoustic waves that may be reflected from the exterior surface of the cladding at other locations along the length of the optical fiber. Accordingly, the irregular lateral thickness of the cladding of the optical fiber of this embodiment further serves to increase the threshold at which stimulated Brillouin scattering commences.
The optical fiber of the present invention therefore provides an increased threshold at which stimulated Brillouin scattering commences. As such, the optical fiber of the present invention can be utilized to deliver optical signals having increased power levels relative to conventional optical fibers and is therefore particularly suitable for applications, such as high power fiber optic industrial lasers and scalable high power fiber optical phased array laser systems, that require the transmission of optical signals having power levels well above the threshold for stimulated Brillouin scattering of conventional optical fibers.